Pre-press color proofing is a procedure used by the printing industry to create representative images of printed material. This procedure avoids the high cost and time required to produce printing plates and set-up a high-speed, high-volume printing press to produce a single intended image for proofing prior to a production run of the intended image. In the absence of pre-press proofing, a production run may require several corrections to the intended image to satisfy customer requirements, and each of the intended images would require a new set of printing plates. By utilizing pre-press color proofing, time and money are saved.
A laser thermal printer having halftone color proofing capabilities is disclosed in commonly assigned U.S. Pat. No. 5,268,708 titled “Laser Thermal Printer With An Automatic Material Supply,” issued Dec. 7, 1993 in the name of R. Jack Harshbarger, et al. The Harshbarger, et al. device is capable of forming an image on a sheet of thermal print media by transferring dye from dye donor material to thermal print media. This is achieved by applying thermal energy to the dye donor material to form an image on the thermal print media. The apparatus disclosed comprises a material supply assembly; a lathe bed scanning subsystem, which includes a lathe bed scanning frame, a translation drive, a translation stage member, and a laser printhead; a rotatable vacuum imaging drum; and exit transports for the thermal print media and dye donor material.
The Harshbarger, et al. apparatus meters a length of the thermal print media in roll form from a material supply assembly. The thermal print media is measured and cut into sheets of the required length, transported to the vacuum imaging drum, and wrapped around and secured to the vacuum imaging drum. Donor roll material is metered out of the material supply assembly, measured, and cut into sheets of the required length. A sheet of dye donor material is transported to and wrapped around the vacuum imaging drum, and superposed in registration with the thermal print media. The scanning subsystem traverses the printhead axially along the rotating vacuum imaging drum to produce the image on the thermal print media. The image is written in a single swath, traced out in a continuous spiral, concentric with the imaging drum, as the printhead is moved parallel to the drum axis.
The Harshbarger et al. apparatus simulates the printing process by imaging the dye donor material at a constant exposure. The dye donor is used to mark or not mark the thermal print media similar to the printing process, which either transfers or does not transfer ink. The apparatus allows the exposure to each dye donor material to be varied over a limited range to allow the customer to match the density of the of the dye deposited on the thermal print media with the density of the ink which will be used to print the image on a press.
Although the printer disclosed in the Harshbarger, et al. performs well, there is a long-felt need to be able to use colors other than the standard cyan, magenta, yellow, and black (CMYK) in pre-press color proofing. The CMYK colors are often referred to as process colors. In the printing industry additional colors, other than cyan, magenta, and yellow, are used depending upon the graphic designers intentions for the printed work. A “key” color may be added to highlight a particular component of the artwork. For screened continuous tone images this key color is typically chosen to be black. The image is modified to use black to adjust the intensity level within the image instead of using cyan, magenta, and yellow together. This is called under color removal. For some work, the customer may choose to use another color, for example, brown, as the key color. This may be appropriate, for example, on a cereal box or in an image with a tan subject. To print the job the printer uses cyan, magenta, yellow and the key color. To save money one or more of the colors may be eliminated. For artwork the printing industry may print the job with the exact inks used by the artist. In these cases the printer may be printing red, blue, or some combination of colors which may or may not include CMYK.
In many cases the color of the subject may not be successfully reproduced using the standard CMYK colorants. In this case an additional color printing plate may be created to be printed with an ink which is a close match to the desired color of the subject. This additional color is imaged with the CMYK layers and is called a “bump” plate. It is important to note that one or more of the process colors may be eliminated or replaced with the bump color. For instance if a red color is used to bump the color of a red car, then the black or cyan process color may be replaced with the red bump color.
In existing pre-press systems, additional donor colors would be needed to accomplish this. For example, commercial systems such as Polaroid Graphics Imaging Polaproof, Dupont Digital Halftone Proofing System, and Imation Matchprint Laser Proof Technology, have all advertised the availability of additional donor colorants to create digital halftone proofs with special colors. This solution, however, requires the manufacturer to produce additional dye donor sheet in special color, often in small volume. Small production runs like this, for one color, are expensive.
Another problem arises when plates in the printing press are out of register. In that case colors are imaged slightly wider and overlapped 440, as shown in FIG. 1, so that a white space error 450, shown in FIG. 2, does not occur. FIG. 2 illustrates an error in alignment between the Kodak Yellow (PMS 123C) 410, and Kodak Red (PMS 485C) 402, color planes, which results in a white space error 450 between the two colors. The printing industry hides this defect by increasing the line width of a color such that errors in color placement are hidden behind the darker color. For example, in FIG. 1, the Kodak Yellow 410 is enlarged to expand behind Kodak Red 402, creating a darker outline 440. This technique is called “trapping.” It is important to be able to see the trapping on each of the printed color planes in the halftone proof. The capability to show trapping is not readily available in state of the art pre-press color proofers without use of a special color dye donor sheet.
Printing presses traditionally uses a halftone screen to generate a tone scale. The printing process is only capable of delivering or not delivering ink, which is usually opaque. This is the binary printing process. To generate a light tint, small dots of ink are used. To generate a darker tint the ink dots are enlarged which touch and fill the space between dots. The halftone proofer disclosed in U.S. Pat. No. 5,268,708 images CMYK colorants at a high resolution. For example, a Kodak Approval XP system produces images at either 2400 dpi or 2540 dpi using a software Raster Image Processor (RIP) to generate a bitmap which determines when the lasers within the printer mark the CMYK films. FIGS. 3, 4, and 5 demonstrate how a halftone image would look at different magnifications. Tone scale is composed of colored dots which are shown in the exploded views in FIGS. 4 and 5. The colors are arranged in a grid and the pitch, dot to dot, is called the screen ruling in dots per inch. The angle of the grid is called the screen angle. Each color is printed at a different screen angle to hide the beating, or aliasing, caused by the alignment and accuracy of the color screens to each other. For optimum conditions the cyan, magenta, and black screens are each separated by 30 degrees. The fourth color, yellow, is then placed at an angle half way between the angle of two of the other colors. In FIGS. 4 and 5 yellow is between the cyan and magenta screens.
The color screens are separated out as planes 630, 640, 650, and 660 in FIG. 6. Cyan plane 630, with a grid 670 superimposed over the halftone dots 631 are comprised of micropixels 632 as shown in FIG. 7. The software RIP determines the positions in the grid when the laser needs to be energized to print the halftone dot. While only cyan plane 630 is shown, a similar computation is performed for the magenta plane 640, the yellow plane 650, and the black plane 660.
It is common practice in color proofing to represent special color planes, i.e. planes containing colors other than the processes colors, by replacing solid color areas with halftone patterns of the process colors as described for example in U.S. Pat. No. 5,309,246. It is usually necessary to attach additional instructions with these proofs to inform customers and printers that a substitution has been made. It is highly desirable for halftone color proofing systems to reproduce the special color planes with colors that more closely represent the final print job. In the case of laser thermal material transfer proofing systems it is well known that this can be accomplished by using individualized donors having the unique color required for the special color plane, however, this process adds additional expense as described above.
FIGS. 8, 9, and 10 demonstrates how the Kodak corporate logo would look at different magnification using conventional halftone screens to approximate Kodak Yellow and Kodak Red inks. FIGS. 9 and 10 are exploded views of the halftone screening near the Kodak “K.” Without special donor colors to image Kodak Yellow and Kodak Red a digital halftone printer can only approximate the finished image that will be printed on press. Also, as shown in FIG. 10, the trapping between colors cannot be clearly shown. A customer may choose either to convert these special colors to process colors, CMYK, and use halftone screens to approximate the color, or may choose to leave these features off the proof. Neither of these solutions is acceptable.